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Background: Psychological interventions are often effective in reducing symptoms of common mental disorders (CMD). However, there are notable within- and between-person differences in how symptoms change over time. To investigate such differences, some studies have used growth curve modelling (GCM) to identify different trajectory classes, allowing for the earlier recognition of distinct patient outcomes through routine outcome monitoring (ROM). These are often associated with various patient characteristics, informing the likelihood of a patient's trajectory during the early phases of treatment, but less is known about how these differ between specific treatments. Aims: The study aimed to investigate whether the trajectory classes identified differed between individuals receiving Cognitive Behavioural Therapy (CBT) and Counselling, using data from the Improving Access to Psychological Therapies (IAPT) programme. Method: The current study employed growth mixture modelling (GMM) to identify trajectory classes of depression and anxiety symptoms among a propensity-score matched sample (N = 10268) to control for pre-treatment confounding factors. Multinomial regression models were fitted to determine the associations between the classes and the two intervention types. Results: Four classes were identified for both depression and anxiety symptom change: (1) Rapid Responders (2) Delayed Responders (3) Low Severity Small Improvers and (4) Non-responders. Participants who received Counselling were less likely to be Rapid Responders according to change in their depression symptoms and less likely to be Delayed Responders with regards to anxiety symptoms. Conclusions: Through recognition of these classes during the delivery of CBT or Counselling, clinicians may be better placed to make treatment decisions that optimise patient outcomes.